1. Field of Invention
The invention relates to a sheet feeder that feeds sheets to an image forming apparatus.
2. Description of Related Art
A sheet feeder of this type is shown in FIG. 11 that includes a sheet pressing plate 101 on which a portion of a front edge side of sheet is stacked thereon, swing arms 102 that have supporting points at an upper portion of the sheet pressing plate 101 and vertically swing a front end of the sheet pressing plate 101, a spring 104 that upwardly urges the front end of the sheet pressing plate 101 against a sheet feed roller 103, and an end guide 105 that regulates rear edges of the sheets, has been known. In this sheet feeder, the front end of the sheet pressing plate 101 is upwardly swung by a pressing force from the spring 104, and the sheets are supplied one by one by rotating the sheet feed roller 103 in a state where the front edge of the uppermost sheet stacked on the sheet pressing plate 10 is pressed against the sheet feed roller 103.
In the sheet feeder, the supporting points of the swing arm 102 function as supporting points of swing of the sheet pressing plate 101. Therefore, a distance between a front edge of the sheet pressing plate 101 and the end guide 105 varies with the vertical swing of the sheet pressing plate 101. Consequently, sheets stacked on the sheet pressing plate 101 may be deviated.
Therefore, an initial proposal was to provide a sheet feeder that prevents the sheets from deviation. Such a sheet feeder is shown in FIG. 12 as a developmental prototype In the developmental sheet feeder, the end guide 105 is urged against the rear edges of the sheets by a spring 106, so that the rear edges of the sheets are pressed by the end guide 105 at all times.
However, the sheets are continuously applied the pressing force from the end guide 105 because the rear edges of the sheets are pressed by the end guide 105 at all times. Therefore, when the stack of sheets accommodated in the sheet feeder is few, the pressing force in this developmental sheet feeder was too strong. As a result, two or more sheets may be fed to the sheet feed roller 103 at a time. On the other hand, when the stack of sheets accommodated in the developmental sheet feeder is large, the pressing force was insufficient. As a result, no sheet may reach and may be fed to the sheet feed roller 103. Consequently, a stable sheet feeding may not be performed as described above.
According to the invention, a sheet feeder that surely and stably feeds sheets to an image forming apparatus, with a simple structure, can be obtained.
In the sheet feeder, a stacking portion where sheets are stacked thereof is swingable vertically, and an end guide where rear edges of the sheets contact moves back and forth in accordance with the vertical swing of the stacking portion. The stacking portion is swung vertically in accordance with a consumption of sheets stacked on the stacking portion.
The stacking portion where the sheets are stacked swings vertically while a distance between the stacking portion and the end guide is maintained at the same distance. Therefore, the sheets stacked on the stacking portion do not move back and forth, so that the sheets are reliably fed one by one at a time, without being fed two or more at a time or failing to be fed.
The end guide is provided on a support portion that supports the rear portions of the sheets. Further, a slit provided to the support portion and a bent portion provided to the stacking portion are engaged with each other. Because the bent portion engages the slit, the support portion moves back and forth when the stacking portion swings vertically. Therefore, the sheet itself can hardly receive the slide resistance due to the back and forth movement, so that the sheet feeding becomes more stable.
If a surface shape of the end guide where the rear edges of the sheets contact is formed such that a middle portion in its vertical direction is curved backward, the front edge of the uppermost sheet stacked on the stacking portion can be positioned at an optimum position where the sheet is always fed regardless of the stacking condition of the sheets, that is, regardless of a swing angle of the stacking portion.
Particularly, in order to distinguish this advantage, the surface shape of the end guide is preferably determined so that the position of the rear edge of the uppermost sheet stacked on the stacking portion always satisfies an equation shown below.
xe2x80x83L+(R1 tan(b+xcex8)xe2x88x92R1 tan xcex8)+(0-5 mm)
(when the rear edge of the stacking portion is positioned ahead of the normal dropping from the center of rotation of the stacking portion)
or
L+(R1 tan xcex8xe2x88x92R1 tan(xcex8xe2x88x92b))+(0-5 mm)
(when the rear edge of the stacking portion is positioned behind of the normal dropping from the center of rotation of the stacking portion)
where:
L: a length of the horizontal component of the uppermost sheet to be stacked on the stacking portion;
R1: a length of a line segment of a normal dropping from a center of swing of the stacking portion to the support portion;
b: a swing angle of the stacking portion from a reference angle xcex8; and
xcex8: an angle formed between the normal dropping from the center of swing of the stacking portion to the support portion and a line segment that connects the center of swing of the stacking portion and the rear edge of the stacking portion when the sheets are fully stacked.